Kirsty Young has achieved a lot in her 39 years, rising without seeming effort through the ranks of TV news-gatherers and presenters, and the starry hierarchy of programme-fronters to be the voice of Desert Island Discs and now the face of Crimewatch, but for much of her career she was The Woman Who Sat On Her Desk. The moment, in 1997, that she first parked her chic, East Kilbride derrière on a studio table to read the Channel 5 News, a chorus of HM Bateman-style mutterings ran round the nation. Standards, people complained, were slipping. British newsreaders were supposed to keep their distance, sit behind their desks, and deliver their bulletins like bank managers discussing a loan. They did not get excited. They did not walk around or wave their hands while apprising the country of the latest threat to its existence. They certainly didn't perch in that forward, chatty, in-your-face manner newly adopted by Ms Young. The Channel 5 News studio's choice of kid's-nursery colours and its jumping disco music also drew protests that the news was becoming "trivialised," but the perching is what stuck in the collective mind. A decade later, when bulletins are routinely delivered by newscasters standing, relaxing on sofas, even hovering in packs around a table like strap-hangers on the Tube, memories of the Kirsty revolution remain.

The fuss may have been an acknowledgement of the stir Ms Young causes by her presence. She was always a little too foxy, too sardonic and too knowing to be a convincing autocutie. Smarter than Selina Scott, sexier than Angela Rippon, sulkier than Fiona Bruce, she had an indefinable attitude, which showed to good effect when she fronted Have I Got News For You on six occasions, and delivered the jokes with a sleek poker face and perfect timing. She was a good newsreader, ice-cool under pressure. She reported the death of Princess Diana on Channel 5 at 5am, and the events of 9/11 on ITV. Her husband Nick Jones, the Soho House impresario, was in New York that day and she found herself interviewing him live on air, shortly after the second plane hit the towers.

"Nobody knew what was going on," she says. "A voice in my ear said, we're going to the Pentagon, there are pictures, and I said to camera, 'Let me show you some pictures from the Pentagon,' and the guys in the studio were trying to interpret the pictures while we were showing them. And I thought, at any moment, they're going to say, 'We're going to Parliament,' and there'd be a plane in the side of the House of Commons. It was the most extreme version of my job that I'll ever do." It was, however, her deep Scots voice and intelligent probing that brought her the jackpot when BBC radio announced in June 2006 that she would shortly take over from Sue Lawley hosting Desert Island Discs.

The Daily Mail, which misses no opportunity to take bitchy swipes at Ms Young, found a BBC staffer who declared: "People are worried about the future of Desert Island Discs, as they see her [Young] as too lowbrow for Radio 4's audience," a sentence which translated as, "Who let this uneducated Scottish bint from commercial telly into the BBC, to steal a job that was obviously meant for Martha Kearney?" Kirsty survived a brief baptism of fire, kicked off her Discs career with Quentin Blake the children's illustrator in October 2006, and was set up with a lucrative career for life. Now, without relinquishing her throne on the sleepy lagoon where slightly over-the-hill celebrities ruminate on their lives, her empire is spreading. From next week she will present Crimewatch, the long-running BBC1 programme in which viewers are invited to grass up armed robbers and heavily disguised fraudsters caught on camera. Its long-running presenter, Nick Ross, had been retired under protest, and his co-presenter, Fiona Bruce, is departing for the Antiques Roadshow. Kirsty Young is in the rare position of piloting two national institutions simultaneously.

We met at the TV centre in Wood Green. Ms Young in the flesh is, basically, Amanda Redman's more classy younger sister, with a touch of Kim Cattrall about the eyes. Her face is broad, and made up as though she's about to go on-air at any moment. Despite her admission, on Room 101, that she would consign cowboy boots to hell, she's wearing long boots today with her legs of her jeans tucked inside. As she talks, she folds one leg beneath her, and extends the other one along the sofa. Cool, friendly and decidedly feline, she's a practised interviewer who gives little away when being grilled herself.

Had she had the Crimewatch job in her sights for a while? Has she, indeed, had ambitions to front other TV shows? (University Challenge? Bargain Hunt?) "No," she says, vehemently. "That way madness lies. I gave up Channel 5 News in August without a clue about what to do next. I wanted to do something else in TV, but I didn't want to sit in one job waiting for another to present itself. I had no idea Crimewatch was ready for refreshing." ("Refreshing" is BBC-speak for "revamping" or "updating.") "It was a surprise when the BBC came and said, 'Are you interested?' and I was flattered but didn't say definitely yes straight away, because it's a live, heavyweight programme." To her credit, she doesn't overdo the piety that tends to hang over the show. "Crimewatch genuinely does make a difference, and that also appealed to me, of course. All of us who work in television sometimes wonder, What's the point of this job? Crimewatch is one of those shows that have a point. Apart from the fact that it pays handsomely, and isn't heart surgery to do."

Why did she think it had endured for 23 years? "It's the format," she said firmly. "What endures in television is the format; it's the same with radio, and Desert Island Discs. If the format's good, people will stick with it, whoever presents it and wherever they move it in the schedules. People are fascinated by crime, and if they feel that the bloke in the flat upstairs is the one who's been robbing every post office in the Midlands, then..." So it's British nosiness? "That's not nosiness, that's doing your civic duty," said Ms Young with a touch of the kirk in her voice. "I always watch the Most Wanted faces absolutely gripped, thinking it could be somebody I know. It could be my accountant, or the bloke I pass in the morning when I'm buying the milk."

She bridles slightly when I ask about the evidence that some viewers tune in just to watch the reconstructions, not always from pure motives. I've met creepy people who'll say, "Is Crimewatch on? I hear there's a very promising rape in the second half..." Was there a moral line top be drawn? "I feel very strongly about this," she said. "Of course there's a moral crux here. There was a show on last year, a drama about a violent crime, where they entirely overstepped the mark and I had to switch it off. I was appalled that somebody who works in television could allow it to be broadcast. I'm not a censorious person; I think people should be allowed to watch what they want. But when it comes to factual programming, there's an enormous responsibility not to titillate." Where would she draw the line? "I think that's for directors and editors to decide. I'm sure it comes down to matters of how long you show something on screen, how real the blood is, how visceral the images. I've not seen every edition of Crimewatch, obviously, but for me it's never transgressed those boundaries."

The "refreshed" show will feature a new opening sequence, new title music, a new set and some new items about psychological profiling, to go with its new presenter. Were they giving her a Severe Clothing Allowance, to express an appropriate gravitas? "I wish they were," she said fervently. But Nick Ross always made sure he wore reassuring jumpers... "Nobody's mentioned it. Maybe it'll be a horrible shock – just before I go on, the first night, they'll hand me a reassuring jumper." Did she have a reassuring bromide with which to end the show, likes Ross's invariable sign-off, "Don't have nightmares"? Ms Young's response was pure Scots nanny: "We'll see."

She was born in November 1968 in East Kilbride. Her father Joe, a policeman, deserted the family when Kirsty was six weeks old and her sister Laura was only a toddler. Her mother married John Young, a Glasgow carpenter, when Kirsty was two. She always speaks warmly of her stepfather. "I've been very fortunate in that I've had two entirely present parents, which is more than a lot of people have – one of them wasn't a biological parent, but he is my parent." East Kilbride was a new town, built towards the end of the Sixties, "like Milton Keynes or Cumbernauld, one of those great white hopes of social engineering, where people live in the same houses, drive the same cars and do the same jobs and there's a uniformity in it, which as a child I really loved. There was a glen across from where we lived, and a hill for sledging. It seemed quite idyllic to me."

It helped that her stepfather ran a newsagents, "with big, old-fashioned jars of sweeties on the shelves". When she was eight, the family moved to Stirling, where she attended the high school. At 11, she experienced a small epiphany. Allowed to stay up late, she watched Newsnight, and was startled to hear a Scottish accent, that of Donald McCormack. They allowed Scottish people on television? Some germ of ambition clearly entered her soul at that moment. She body-swerved university, preferring to work as an au pair in Spain and Switzerland. At 21, she made her first broadcast – a continuity announcement on Radio Scotland. Three years later, in 1992, she joined STV as a presenter on Scotland Today. And there she began to read the news, accessorised with big earrings and big shoulders. When an afternoon chat show in 1996 (Kirsty) failed to take off, she headed south to England, Channel 5, Discs, Crimewatch and a stake in broadcasting history.

When Nick Ross left the show last June after 23 years, his contract unrenewed, accusations flew that the BBC was pursuing an ageist agenda: Ross was pushing 60. The veteran newsreader Moira Stuart had recently been axed from the Andrew Marr show when she hit an almost equally ancient 55. "They brought in outside producers," Ross complained, "and asked them 'what do you think we should do to the show?' I think they said, 'You are not going to make it look very different if Nick is still fronting it.'" He said the producers were "patronising in the extreme" to think audiences wanted to see only young faces presenting such shows. So, had Kirsty been brought in to appeal to crimewatching youth? "I was perplexed by that. I mean, Fiona Bruce is only a couple of years older than me. If it were indeed the case that they were looking for a bit of totty, they might have looked beyond somebody who was 39. So I think, that's cobblers." Had anyone talked to her about appealing more to youth? "Nobody's said, 'Dress younger, look younger, get your eyes done.' Maybe they will in 10 years. I shall wait with interest for that." So she wasn't planning to get down with the kids in the 'hood? "That would be ludicrous. They wouldn't get me if they wanted somebody to be hangin' with the homies."

Yeah, blood. Ms Young does a rather good urban jive accent. She turns out, behind all her guardedness, to be a good mimic of celebrity voices. She does a brilliant Yoko Ono, clearly her favourite guest to date on Desert Island Discs. "I'd become aware of Yoko only when John Lennon died. I wasn't old enough to have an opinion about her running off with a Beatle. I did all the research and wasn't sure what to expect. I thought she might swan in with a huge entourage. She was extraordinary. She wore this sweeping neckline, with this magnificent bosom on display – at 74! – and kept her sunglasses on for the entire interview, though she did sometimes peer over them. She was not, as my grandma would say, the height of tuppence. And 10 minutes into the interview, she just seemed to decide, I'm going to say what I think." Ms Young looks charmingly shocked. "She talked about her dealer, about the difficulty of getting good drugs." Her voice affects a convincing Japanese whine: "We never had good stuff. We'd go to other people's parties and say, 'This shit is great, what's wrong with our stuff?'" Young was also startled to hear the empress of the avant-garde discuss "a conversation she'd had with John Lennon about whether he wanted her to abort their child. She had never said that before. Sometimes she just talked nonsense, and then suddenly she'd be incredibly lucid and thoughtful. She said about her art, 'Nobody would pay me for this stuff,' and I thought, Yes, that's an incredibly honest comment."

Kirsty Young has ruled over the desert island for a year, and teased out the life stories of, among others, Camila Batmanghelidjh (whose chose a yoyo as her luxury), Brian Aldiss (who included in his eight a traditional Yugoslav folk song called "Oh My Darling") and Professor Sir Alan Jeffreys, the father of DNA fingerprinting, who confessed a passion for banging Ibiza dance music and selected a track by Jah Rule. "And he wanted to have 'The Laughing Policeman' because, he said, he'd left so many policemen with smiles on their faces," said Kirsty, fondly. "I had to forgive him for that, he was such a nice man." She was less successful with Paul Weller, who sounded throughout his interview like a mutinous teenager trapped by a posh headmistress ("I think the magnitude of his talent is in his work, and that's how he expresses himself," explained Kirsty, kindly.) She was told by Sue Lawley not to underestimate how much research she'd have to put in every week. "You need to understand something of the person before you go into the studio so that, when someone says, 'I did this in 1973,' you can say, 'Yes, that was when you wrote this book, or when your marriage broke down'." Was there anything about the show that surprised her? "Only the fact that, wherever I go, people talk about it all the time. Always. They'll refer back to programmes they've heard, or wonder why so-and-so hasn't been invited on. I mean, I've listened to it all my life, and I know it has millions of listeners, but I didn't realise they'd be so... preoccupied." She laughed. "I told that to Stuart Rose who runs Marks & Spencer. He said, wherever he goes, people only want to talk to him about knickers."

Which guests are on her personal wish list? "Kate Moss would be good, because we're never really heard her speak. The Queen is definitely on my list. And Camilla – I've met her sister, who's rather fabulous, and I think she'd be great. Hillary Clinton if she gets the nomination, though she has so much to lose right now, she might be a bit tight and wooden. Bill would be an interesting person to do right now, much more than when he was trying to define sexual intercourse in front of an impeachment committee."

And so the divine Ms Young, benign interrogator of the good, the great and the globally famous, casts an Olympian eye on the names she may choose to honour with a slot on her show. It's a far cry from East Kilbride, and her stepfather's newsagents shop with the old-fashioned sweetie jars. One of the few things she's taken with her from those days is her voice, which, she says, hasn't changed a lot from childhood. "I've always had a deep voice, since puberty. It was never very Glaswegian, because we moved to Stirling when I was eight, and that made it more central-Scots. My mother has a beautiful voice and accent, and was a stickler for pronunciation and diction. That's what I was most exposed to." And, of course, it's been her fortune. "When I first came down here," she recalls with a sardonic raise of the eyebrow, "I met a producer at the Channel 5 launch party, and told her what I was doing, and she said, 'Are you going to do it in that accent?' I said, 'Well, it's the only one I have...'"